The paper used as the filtration medium for automotive type filters has been traditionally treated with a phenolic resole type resin. This has been done to improve the paper's strength properties and allow it to be pleated in an accordian like shape and to hold this shape when the resin is cured. The standard phenolic resin used to treat automotive filter paper have had relatively low mol ratios, i.e., 1.0/l to 1.3/1 formaldehyde/phenol. This has been necessary so that good final paper properties, especially flexibility, can be achieved. Higher mol ratio resins tend to result in brittle paper when they are cured.
The traditional method of making an automotive type filter has been for a paper maker to treat a base filter sheet with an alcohol solution of these phenolic resole resins. The treated sheet is passed through an oven to drive off the solvent and make a so-called B-staged sheet. This sheet is then shipped to the filter maker where it is pleated and put through an oven to further cure the resin and therefore hold the shape of the pleats.
With the onset of the energy crisis many of the filter manufacturers have requested the paper suppliers to supply a sheet that holds its pleats, needs no high temperature cure thus eliminating the final curing step and the fuel needed to run the curing ovens and still meets all of the requirements of the previous paper.
One approach to this has been for the filter paper manufacturer to cure the sheet saturated with the traditional low mol ratio resins during the B-staging process. This has several disadvantages. First, these low mol ratio resoles are relatively slow curing so that the line speed of the treater is uneconomically low or the treater temperatures very high so that much more fuel is used to achieve the desired degree of cure. A second disadvantage is that these sheets when cured cannot be pleated. They are brittle and crack during the pleating operation. These cracks are obviously undesirable for a filter. The sheet can also be very stiff and difficult to handle when it is put in the metal filter canister or other filter.
It has been found that by using a polyvinyl acetate resin in combination with a relatively high mol ratio phenolic resole, several advantages result. First, the polyvinyl acetate appears to synergistically increase the rate of strength development of the already faster curing high F/P resin as measured by development of filter paper properties. Secondly, the polyvinyl acetate resin plasticizes the brittle high F/P resole and allows paper that has been cured to be pleated without cracking. This paper is also more flexible under the conditions of use and therefore withstands the stress in an oil filter better. A further advantage is that the high F/P resoles contain less of the monomers that volatilize off during the B-staging and curing operations therefore reducing air pollution.